The following information describes the Modbus Application Protocol (MBAP).
The Modbus Application Protocol (MBAP) is a layer-7 protocol providing peer-to-peer communication between programmable logic controllers (PLCs) and other host-based nodes on a LAN. Collectively, these nodes implement all or part of a control application used for industrial automation applications in the automotive, tire and rubber, food and beverage, and utilities industries, to name a few
Client-Server Communications
Modbus protocol transactions are typical request-response message pairs between a client node and a server node. These nodes function as follows
Client
: The node that initiates a data transaction is called a client. The Modicon Quantum Ethernet module provides the user with the capability to transfer data to and from nodes on a TCP/IP network using a communication instruction. All PLCs that support networking communication capabilities over Ethernet can use either the MBP_MSTR Ladder Logic instruction to read or write controller information or IEC communication blocks.
Server
: The node that receives an inquiry is the server. Using the standard Modbus/TCP protocol, the Modicon Quantum Ethernet module provides access to controller data. Any device, PC, HMI package, another PLC, or any Modbus/TCP compliant device can access data from the PLC. The Modbus/TCP server also allows programming panels to log into the controller over Ethernet.
Modbus requests contain function codes representing several classes of service including data access, online programming, and program download and upload classes. Modbus responses can be with and without data, or NACKs with error information.
The Modbus Application Protocol can be transmitted over any communication system that supports messaging services. However, the current Quantum implementation transports Modbus Application Protocol PDUs over TCP/IP. The Quantum PLC accommodates both , although Ethernet II framing is the default.
The Modicon Quantum Ethernet module supports up to 64 simultaneous Modbus/TCP server connections. To guarantee consistency of changes to the controller configuration, the module allows only one programming panel to be logged in at a time.
The module supports these Modbus/TCP commands:
